Initial commit: hardened DeerFlow factory

Vendored deer-flow upstream (bytedance/deer-flow) plus prompt-injection
hardening:

- New deerflow.security package: content_delimiter, html_cleaner,
  sanitizer (8 layers — invisible chars, control chars, symbols, NFC,
  PUA, tag chars, horizontal whitespace collapse with newline/tab
  preservation, length cap)
- New deerflow.community.searx package: web_search, web_fetch,
  image_search backed by a private SearX instance, every external
  string sanitized and wrapped in <<<EXTERNAL_UNTRUSTED_CONTENT>>>
  delimiters
- All native community web providers (ddg_search, tavily, exa,
  firecrawl, jina_ai, infoquest, image_search) replaced with hard-fail
  stubs that raise NativeWebToolDisabledError at import time, so a
  misconfigured tool.use path fails loud rather than silently falling
  back to unsanitized output
- Native client back-doors (jina_client.py, infoquest_client.py)
  stubbed too
- Native-tool tests quarantined under tests/_disabled_native/
  (collect_ignore_glob via local conftest.py)
- Sanitizer Layer 7 fix: only collapse horizontal whitespace, preserve
  newlines and tabs so list/table structure survives
- Hardened runtime config.yaml references only the searx-backed tools
- Factory overlay (backend/) kept in sync with deer-flow tree as a
  reference / source

See HARDENING.md for the full audit trail and verification steps.

.gitignore

@@ -0,0 +1,78 @@
+# ============================================================================
+# Secrets — NEVER commit
+# ============================================================================
+.env
+.env.local
+.env.*.local
+secrets/
+*.pem
+*.key
+!**/test*.pem
+!**/test*.key
+
+# ============================================================================
+# Python
+# ============================================================================
+__pycache__/
+*.pyc
+*.pyo
+*.pyd
+.venv/
+venv/
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
+*.egg-info/
+.coverage
+htmlcov/
+
+# ============================================================================
+# Node / pnpm / npm
+# ============================================================================
+node_modules/
+.pnpm-store/
+.next/
+dist/
+build/
+*.log
+
+# ============================================================================
+# Editor / OS
+# ============================================================================
+.vscode/
+.idea/
+.DS_Store
+Thumbs.db
+*.swp
+*.swo
+*~
+
+# ============================================================================
+# DeerFlow runtime
+# ============================================================================
+deer-flow/.deer-flow/
+deer-flow/backend/.deer-flow/
+deer-flow/backend/uploads/
+deer-flow/backend/checkpoints/
+deer-flow/backend/threads/
+deer-flow/docker/.cache/
+deer-flow/.omc/
+
+# Local DeerFlow config (overlay) — only the example/template lives in git
+deer-flow/config.yaml
+deer-flow/backend/config.yaml
+deer-flow/mcp_config.json
+deer-flow/extensions_config.json
+
+# Quarantined upstream tests for disabled native providers
+# (kept on disk but excluded from collection — see conftest.py inside)
+# We DO want them in git so the disable trail is visible.
+!deer-flow/backend/tests/_disabled_native/
+
+# ============================================================================
+# Backups / scratch
+# ============================================================================
+*.bak
+*.backup
+*.tmp
+.scratch/